The use of photoinitiators to bring about the polymerization of free radically polymerizable compounds and compositions is well known and many photoinitiators are commercially available. The selection of a particular photoinitiator for use in a composition is generally made on the basis of the solubility, rate of reaction, activating wavelength, and intended use of the photoinitiator (e.g., use as protective coatings, viscoelastic products, and the like).
Until recently, photoinitiators have been radiation sensitive compounds that, on exposure to activating radiation of monomer compositions containing them, will in the absence of polyfunctional monomers in the composition induce polymerization of the monomers in a composition to essentially linear thermoplastic polymers. Included among these initiators are acyloin and derivatives thereof, e.g., benzoin, benzoin methyl ether, benzoin ethyl ether, benzoin isopropyl ether, benzoin isobutyl ether, and .alpha.-methylbenzoin, organic sulfides, e.g., diphenyl sulfide, diphenyl disulfide, decyl phenyl sulfide, and tetramethylthiuram monosulfide, S-acyl dithiocarbamates, e.g., S-benzoyl-N,N-dimethyldithiocarbamate, and .alpha.-substituted acetophenones, e.g. .alpha.,.alpha.-dimethyl-.alpha.-hydroxy acetophenone, .alpha.,.alpha.-diethoxy acetophenone. The initiator is generally used in amounts ranging from about 0.01 to 5.0% by weight of the total polymerizable composition.
Photosensitive compounds are known that when incorporated into photopolymerizable compositions bring about the crosslinking of the composition with attendant enhancement of the cohesive strength of the composition. Examples of these photoactive compounds include: sulfonyl halides, such as .beta.-naphthalene sulfonyl chloride; halogenated aromatic compounds, such as .alpha.-chloromethylnaphthalene; and trichloromethyl-s-triazines, such as 2,4-bis(trichloromethyl)-6-(4-methoxystyryl)-s-triazine. These photosensitive compounds, although very effective for use in the polymerization and photocrosslinking of monomer containing compositions liberate hydrogen chloride as a by-product, following hydrogen abstraction by chlorine radicals which are responsible for initiating the polymerization and crosslinking. The corrosiveness of the liberated hydrogen chloride makes these photosensitive crosslinkers unsatisfactory for some purposes.
Photosensitive compounds that can bring about the crosslinking of monomer containing compositions without the liberation of hydrogen chloride are disclosed in European Patent Application No. 0 281 941. In this application, coreactive photoinitiators are disclosed that have the general formula: EQU RG--A--IN I
in which IN denotes a photoinitiator structure, A denotes a spacer group, and RG denotes a functional group. This extremely broad Formula I includes acryloyloxy functional photosensitive compounds and acrylamidoacyl functional photosensitive compounds such as ##STR1## 2-propenoic acid, 2-[4-(2-hydroxy-2-methylpropanoyl) phenoxy]ethyl ester II, also called 2-hydroxy-1-[4-(2-acryloyloxyethoxy)phenyl]-2-methyl-1-propanone) and ##STR2## 2-propenoylaminoethanoic acid, 2-[4-(2-hydroxy-2-methylpropanoyl)phenoxy]ethyl ester, III.
Acryloyloxy functional photosensitive compound II is also disclosed in RADCURE '86, Conf. Proc., 10th, 4/43-4/55 (Sep. 8-11, 1986); and its preparation in U.S. Pat. No. 4,922,004, among other patents in which it is taught that Compound II is prepared in a calculated 60% yield from 2-hydroxy-1-[4-(2-hydroxyethoxy)phenyl]-2-methyl-1-propanone (Irgacure.TM. 2959, Ciba-Geigy, Chicago, Ill.) by reaction with acryloyl chloride.
Acrylamidoacetyl functional photosensitive compounds such as Compound III are included in the EP application 281,941 formula RG-A-IN only by proper combination of groups from among the many groups disclosed as useful in the formula; the formula discloses for spacer group "A" only unsubstituted (CH.sub.2) units.
Japanese KOKAI Patent HEI 2[1990]-248482 describes a photocurable pressure sensitive adhesive (PSA) suitable for screen printing enabling a high rate of production of printed circuit boards using as photoinitiator 1-acryloyloxy-2-[4-(4-chlorobenzoyl)benzoyloxy]ethane, a hydrogen abstracting photocrosslinker. Such a composition would yield an adhesive having a shear value of less than 100 minutes.
Japanese KOKAI Patent HEI 2[1990]-235908 discloses pressure sensitive adhesives by use of a composition of ethylenically-unsaturated cleavage type photopolymerization initiator such as 2-hydroxy-1-[4-(2-acryloyloxyethoxy)phenyl]-2-methyl-1-propanone [Formula II], a conventional photopolymerization initiator and a polyfunctional crosslinking agent such as hexanediol diacrylate. The PSA would have good shear primarily because of the conventional polyfunctional crosslinking agent. In the absence of such a conventional polyfunctional crosslinking agent the PSA would be expected to exhibit poor shear.